All communications systems rely on a precise reference oscillator to determine the output frequency of the system. The reference oscillator is typically a crystal oscillator. A typical crystal oscillator has an accuracy of approximately ±30-40 parts-per-million (ppm). However, communications standards, such as the Global System for Mobile Communications (GSM) standard, require a reference oscillator with an accuracy of at least ±0.02 ppm. In order to obtain the desired accuracy, the frequency of the crystal oscillator is typically corrected by warping the crystal with a varactor diode variable capacitance driven by a precision Digital-to-Analog (D/A) converter or by controlling individual capacitive elements in a precision capacitor array.
One issue with the above system that significant factory calibration of either the precision D/A converter controlling the capacitance of the varactor diode or the precision capacitor array must be performed. Further, even after calibration, the control of the D/A converter or capacitor array is not precise, and, as such, the frequency correction algorithm must make multiple iterations to accurately correct the frequency of the reference oscillator.
One alternative is to purchase a relatively expensive temperature compensated crystal oscillator (TCXO) sub-assembly for the communications system. However, even for these TCXO sub-assemblies, frequency correction is typically required to obtain the desired accuracy and precision.
Thus, there remains a need for a system and method for correcting or compensating for a frequency error of a reference oscillator.